1. Field of the Invention
The present invention generally relates to a ceramic circuit board and a method for producing the same. More specifically, the invention relates to a reliable ceramic circuit board having an excellent thermal shock resistance, a method for producing the same, and a reliable power module using the ceramic circuit board.
2. Description of the Prior Art
As one of substrates used for mounting thereon electronic parts, there is utilized a ceramic circuit board wherein a metal plate is bonded to at least one surface of a ceramic substrate. Ceramic substrates are particularly utilized as substrates for large power elements, such as substrates for power modules, since the withstand voltage and thermal conductivity of ceramic substrates are superior to those of glass epoxy substrates and metal substrates. In recent years, power modules are mounted on ceramic substrates for controlling motors for automobiles and electric trains, so that the higher reliability of ceramic circuit boards is requested.
As improved reliable ceramic circuit boards complying with such a request, ceramic circuit boards using an aluminum plate as a circuit metal plate and/or a heat sink metal plate (Japanese Patent Laid-Open Nos. 7-276035 and 11-263676), a ceramic circuit board using a ceramic having an improved strength (Japanese Patent Laid-Open No. 2001-77245), and ceramic circuit boards using a metal plate having a sectional shape of a stress relaxation structure (Japanese Patent Publication No. 7-77989 and Japanese Patent Laid-Open No. 3-261669) have been proposed to be put to practical use. These ceramic circuit boards are designed to avoid defectives due to dielectric breakdown and so forth, by suppressing the occurrence of cracks in a ceramic substrate when thermal shocks (heat cycles) are applied thereto. There have been also disclosed a ceramic circuit board having a ceramic substrate having a controlled initial warpage (Japanese Patent Laid-Open No. 11-330308), a ceramic circuit board having a defined flexure quantity when a load is applied to a ceramic substrate (Japanese Patent Laid-Open No. 7-202073), and a ceramic circuit board having a metal plate having holes in the vicinity of the periphery thereof (Japanese Patent Laid-Open No. 5-41566). These ceramic circuit boards have an improved reliability, such as an improved heat cycle resistance.
The ceramic circuit board, which uses the aluminum plate and which is disclosed in Japanese Patent Laid-Open Nos. 7-276035 and 11-263676, of the above described conventional ceramic circuit boards, can decrease stress applied to the ceramic substrate by the plastic deformation of the aluminum plate, so that its reliability, such as heat resistance, is superior to that of a ceramic circuit board using a copper plate or the like. However, if the aluminum plate is thicker or if the aluminum plate is bonded to the ceramic substrate by the brazing and soldering, there are some cases where stress is too strong to obtain sufficient reliability.
If the ceramic having a high strength, i.e., a high bending strength and/or toughness, which is described in Japanese Patent Laid-Open No. 2001-77245 is used, material costs are often higher than those in general articles due to the expensiveness of the material and difficulty of production, and other characteristics, such as thermal conductivity, are often deteriorated. Therefore, it is desired to provide a ceramic circuit board having a satisfied reliability even if a general alumina or aluminum nitride substrate, which is mass-produced by the sheet forming method or the like and which has an average bending strength of about 20 to 50 kg/mm2 at present, is used. Even in the case of a ceramic substrate having a high strength, such as a silicon nitride substrate, there are some cases where the thickness of the substrate is decreased by the request to improve thermal conductivity or the like. In such cases, there are some cases where a sufficient reliability can not be obtained.
The ceramic circuit boards which are disclosed in Japanese Patent Publication No. 7-77989 and Japanese Patent Laid-Open No. 3-261699 and which use a metal plate having a sectional shape of a stress relaxation structure, can obtain some advantageous effects with respect to stress relaxation, but production costs for forming such a structure (shape) are higher than those for usual products which do not have such a structure.
The ceramic circuit boards which are disclosed in Japanese Patent Laid-Open Nos. 11-330308 and 7-202073 and which has a controlled initial warpage or flexure quantity, can obtain some advantageous effects. However, it has been found that the initial warpage or flexure quantity does not always correspond to a thermal shock resistance during assembly and that there are some cases where cracks are produced during heating or cooling when a chip is soldered to the substrate.
The ceramic circuit board which is disclosed in Japanese Patent Laid-Open No. 5-41566 and which has a metal plate having holes in the vicinity of the periphery thereof, can obtain some advantageous effects without increasing production costs. However, in recent years, it is desired to further increase the thermal shock resistance by shortening the time required to carry out an assembly step. In addition, there are some case where cracks are produced in accordance with the structure of the ceramic circuit board, e.g. the material and thickness of the metal plate and the strength of the ceramic substrate, although it is superior to a ceramic circuit board having a structure having no holes. Therefore, there are some cases where the ceramic circuit board can not be used if only this technique is used.
It is therefore an object of the present invention to eliminate the aforementioned problems and to provide an inexpensive, reliable ceramic circuit board, a method for producing the same, and a reliable power module using the ceramic circuit board.
In order to accomplish the aforementioned and other objects, the inventors have diligently studied and found that it is possible to provide an inexpensive, reliable ceramic circuit board by controlling the warpage of the ceramic circuit board at 350xc2x0 C., i.e. the warpage of the ceramic circuit board at a high temperature when chips, terminals and/or a heat sink plate are soldered, and the warpage of the ceramic circuit board when the temperature thereof is returned to a room temperature, in order to improve there liability of the ceramic circuit board against thermal shocks at a subsequent step of assembling a power module. That is, the inventors found that it is possible to improve the thermal shock resistance of a ceramic circuit board, e.g., it is possible to prevent cracks from being produced during assembly, by particularly controlling the warpage of the ceramic circuit board at a high temperature, and that it is possible to improve the heat cycle resistance of the ceramic circuit board as a product by controlling the warpage of the ceramic circuit board when the temperature thereof is returned to a room temperature. Thus, the inventors have made the present invention.
According to one aspect of the present invention, a ceramic circuit board comprises: a ceramic substrate; and a metal circuit plate bonded to one surface of the ceramic substrate, wherein assuming that a warpage of the ceramic circuit board is a difference in height between a center and edge of the metal circuit plate and is positive (+) when the circuit board warps so as to be concave on the side of the metal circuit plate, the ceramic circuit board has a warpage of xe2x88x920.1 to +0.3 mm when the ceramic circuit board is heated to 350xc2x0 C., and has a warpage of +0.05 to +0.6 mm when the temperature of the ceramic circuit board is returned to a room temperature after the ceramic circuit board is heated to 350xc2x0 C.
In this ceramic circuit board, the ceramic circuit board may have an initial warpage of +0.05 to +0.6 mm. The ceramic circuit board preferably further comprises a metal plate bonded to the other surface of the ceramic substrate. In addition, at least one through hole or recessed portion is preferably formed in a surface of at least one of the metal circuit plate and metal plate in the vicinity of the periphery thereof. The metal circuit plate and the metal plate are preferably made of copper or aluminum as a principal component. If the principal component of the metal circuit plate and metal plate is copper, the thickness of the metal circuit plate is preferably in the range of from 0.2 mm to 1.2 mm, and the thickness of the metal plate is preferably in the range of from 0.1 mm to 1.1 mm. If the principal component of the metal circuit plate and metal plate is aluminum, the thickness of the metal circuit plate is preferably in the range of from 0.3 mm to 2.4 mm, and the thickness of the metal plate is preferably in the range of from 0.1 mm to 2.2 mm. The principal component of the ceramic substrate is preferably selected from the group consisting of alumina, aluminum nitride and silicon nitride. The ceramic substrate preferably has a three-point bending strength of 30 kgf/mm2 or more.
According to another aspect of the present invention, there is provided a method for producing a ceramic circuit board, the method comprising the steps of: preparing a ceramic substrate and a metal circuit plate; and bonding the metal circuit plate to a concave side of the ceramic substrate. In this method for producing a ceramic circuit board, the metal circuit plate and metal plate are preferably bonded directly to the ceramic substrate or bonded to the ceramic substrate via a brazing filler metal. In addition, the metal plate is bonded to a convex side of the ceramic substrate.
According to a further aspect of the present invention, there is provided a power module assembled by using any one of the above described ceramic circuit boards.